In recent years, particularly with increasing use of the internet, there has been an interest in providing higher data rates to users. One objective of developments in this area, in countries with a heavy historical investment on copper access networks, has been to make use of existing twisted-pair telephone lines. A result of this has been the Asymmetrical Digital Subscriber Line (ADSL) approach in which it was found that an existing copper pair from a telephone exchange or central office to a telephone subscriber's premises could, using suitable modulation techniques, support significant downstream data rates, of the order of 1.5 Mbit/s. However the actual rate obtained in practice depends on the quality and length of the path from the exchange. An alternative proposal, providing higher data rates, is to make use of the copper pair only from some point rather closer to the user, normally from the intermediate node or cabinet connected to the exchange by an optical link. This is sometimes referred to as a “fibre to the cabinet” arrangement, and is used with very high speed Digital Subscriber Line (VDSL) technology or other Digital Subscriber Line technologies, which are generally referred to as “xDSL”. xDSL signals, produced using xDSL modulation (rather than SDH or ATM techniques) are referred to herein as “broadband signals” and are discussed further in appendix 1.
In one type of “fibre to the cabinet” arrangement, a broadband service is provided from the exchange by a multiplexer/demultiplexer which multiplexes signals, using ATM or SDH techniques, onto one or more optical fibres that feed the cabinet. Within the cabinet there is provided (for each fibre) an ATM or SDH demultiplexer, and at least one DSL access multiplexer (DSLAM) for providing, on the basis of the demultiplexed ATM or SDH signals, broadband signals suitable for travelling over the copper pairs, which ultimately lead to the subscriber premises. The or each xDSL modem is connected via filters to the copper pairs so that the broadband signals can be carried over a different frequency range to that of the telephony signals, without the telephony signals and the broadband signals affecting one another. A power supply is normally provided in the cabinet to supply power to the various optical/electrical components.
Because of cross talk and other undesirable effects, is necessary to tailor the power spectrum of broadband signals to the characteristics of the particular line which is to carry them. Each line has a constraint profile associated therewith (know empirically from tests on the line), specifying the maximum allowed power at each frequency. Typically, a constraint profile will have at least one spectral region in which the allowed spectral power is diminished relative to adjacent regions.
It has been suggested to provide xDSL modulator equipment at the exchange or other location feeding the exchange, such that the xDSL-modulated signals can be transmitted from the exchange over an optical extension link to the cabinet before being launched onto a telephone line. However, the transmission over the link introduces noise. This noise can be particularly troublesome in the spectral region(s) where the broadband signal is low as a result of the restrictions on the maximum allowed power imposed by the constraint profile of the relevant line.